Oleo-pneumatic dampers are known, of the type comprising first and second air chambers, the first chamber being a low-pressure chamber and the second being a high-pressure chamber, for example in aircraft undercarriage trains. Such a damper is provided with two separating pistons, and the development of the load in dependence on the travel in the compression movement of the damper follows a curve, referred to as the polytrope of the damper, having two portions which are separated by a deflection point and each of which represents a rising function. Thus, the first portion of the curve corresponds to the rise in pressure in the low-pressure chamber under the effect of displacement of one of the separating pistons, resulting from the loading applied to the damper. When the pressure in the low-pressure chamber has reached the level of the pressure in the high-pressure chamber, the load development follows the second portion of the curve, corresponding to the rise in pressure in the two chambers under the effect of the displacement of the two separating pistons, resulting from the continuing loading being applied to the damper.
However, the use of such dampers on aircraft with rotating aerofoils, for example helicopters, has shown that the relatively low-frequency operating vibrations developed by such rotating acrofoils when the aircraft is still on the ground can give rise to ground resonance phenomena which may be particularly dangerous when the aircraft is at the limit of lift, since such phenomena can result in a loss of stability of the aircraft, and the aircraft can suffer damage or even destruction as a result. A substantial cause of the known dampers being unsuited for use on aircraft with rotary aerofoils, which are subjected to ground resonance phenomena, is the existance of an non-negligible damper loading threshold, before a compression stroke movement of the damper occurs.
In addition, the above-mentioned dampers have been optimized either for operation at a low speed of compression movement of the damper, as occurs for example when an aircraft is taxiing on the ground, or for operation at a high compression speed, as occurs when an aircraft lands, at the moment of touching down on the ground, at a substantial vertical speed.